1. Field of the Invention
The invention relates to diaphragms used in the alkaline water electrolysis. More particularly, this invention relates to an improved nickel oxide based diaphragm and a method for producing the same.
2. Discussion of the Prior Art
In general, the alkaline water electrolysis was effected at relatively low temperatures (below 90.degree. C.). It has been necessary to employ such temperatures due to the low chemical stability of the asbestos diaphragms normally used in hot KOH. These low temperatures are both thermodynamically and kinetically disadvantageous. As a result, unnecessarily high electrolysis voltages are required and the whole process is uneconomical on energetic grounds.
For this reason, there has been a long felt need either to improve the stability of asbestos in hot KOH or to find other diaphragm materials.
Thus, potassium silicate has been added to the KOH electrolyte in order to reduce the solubility of asbestos in KOH (R. L. Vic et al. in "Hydrogen Energy Progress" IV, 4th WHE Conference, June 13-17, 1982, California, pages 129-140). It is evident that this measure cannot be looked upon as being entirely satisfactory.
The same authors also employed a diaphragm of teflon-bound potassium hexatitanate which was originally developed by the Energy Research Corporation (see also M. S. Casper, "Hydrogen Manufacture by Electrolysis, Thermal Decomposition and Unusual Techniques", Noyes Data Corp., Park Ridge, 1978, p. 190). This diaphragm is, however, somewhat expensive and the voltage drop stemming from the diaphragm is comparable with that of the asbestos diaphragm (see M. S. Casper supra).
Described in the International Journal of Hydrogen Energy, 8, (1983), pages 81-83, is another separator for use in alkaline water electrolysis, which separator uses polyantimonic acid bonded with polysulfone and acts as an ion exchanger. This separator is still in the development stage and is not, therefore, available. A serious drawback associated with this separator is, in any event, its high electrical membrane resistance of 1.0 to 0.8 ohms.multidot.cm.sup.2 at room temperature.
Consequently, other diaphragms with a lower electrical resistance were produced as, for example, a diaphragm comprising a sintered oxide ceramic (J. Fischer, H. Hofmann, G. Luft and H. Wendt: Seminar "Hydrogen as Energy Vector" Commission Europ. Comm., Oct. 3-4, 1978, Brussels, pages 277-290). While this diaphragm is distinguished by its very good electrical resistance (0.027 to 0.27 ohms.multidot.cm.sup.2 at 25.degree. C.), its production is not simple and requires: (i) the production of a suitable oxide material such as ZrO.sub.2, BaTiO.sub.3, K.sub.2 Ti.sub.6 O.sub.13, etc., which is effective as the main component of the porous layer, and (ii) the sintering together of the powder at high temperatures in the range between 1300.degree. C. and 1700.degree. C.
Further, proposals have been made to produce porous metal diaphragms from sintered nickel (P. Perroud and G. Terrier: "Hydrogen Energy System", Proc. 2nd WHE Conference, Zurich 1978, page 241). These have a very low electrical resistance and are also mechanically stable and inexpensive. The great drawback encountered in these diaphragms resides in the fact that, like the electrodes, they are also electron-conducting and as a result, with a compact form of construction geometry, there is too great a danger of a short-circuit.
In order to overcome the aforedescribed problems encountered due to electron conductivity, the inventors have developed porous nickel oxide diaphragms which are obtained by the oxidation of sintered metal at an elevated temperature as taught in U.S. Pat. No. 4,394,244 or, more simply, by the oxidative calcination of a nickel powder layer pressed on to a support as taught in U.S. Pat. No. 4,356,231. These Ni oxide diaphragms pose outstanding properties as separators for the alkaline water electrolysis process. The contents of the aforementioned U.S. Pat. Nos. 4,394,244, and 4,356,231 are incorporated herein by reference as if set forth herein in full.
The diaphragms obtained by these simplified production methods have since been used repeatedly in the most varied electrolysis investigations and have proven to be successful. Thus a check was made of their long-term stability in the alkaline water electrolysis process, the longest testing period until now being over 8000 hours at 120.degree. C. The diaphragms were still intact after this period of use. To be sure, thermodynamic considerations suggest that, after a sufficiently long time, these diaphragms could be reduced to nickel, on the cathode side, either by the cathode itself or by the hydrogen which is produced. Opposing this thermodynamically conditioned effect is only a kinetically conditioned restraint which must diminish after a hitherto unknown time. While this can be fully adequate for the purpose of a water electrolysis, there remains, however, some level of uncertainty.
The following test shows that these considerations are correct:
A diaphragm prepared in accordance with U.S. Pat. No. 4,356,231 was exposed to a hydrogen atmosphere at 200.degree. C. In the process, a gradual reduction of the NiO to Ni was observed which suddenly increased after 1500 hours, so that after 2000 hours the entire NiO content was completely reduced.
This reduction actually proceeds much more slowly in the temperature range 140.degree. to 170.degree. C., but it is still appreciable, however, as may be seen from FIG. 1. After 2000 hours, 7% of the oxygen contained in the NiO has been removed. (Stabilization sets in after about 4500 hours, in which case about 10% of the oxygen will have been removed).
Ceramic diaphragms made from thermodynamically-stable oxides such as, for example, ZrO.sub.2, BaTi.sub.3, K.sub.2 Ti.sub.6 O.sub.13, etc., (see above) do not undergo such a reductive attack by hydrogen. However, the production of such diaphragms is associated with the drawbacks already described above, especially with very high production temperatures, and are attacked in the course of time in 10 N KOH at elevated temperatures.
On the other hand, the NiO diaphragm, produced "in situ" in accordance with the U.S. Pat. No. 4,356,231, is lye-resistant and its production not only involves the use of an inexpensive starting material, but also offers the decisive technological advantage in that the exothermic reaction EQU 2Ni+O.sub.2 .fwdarw.2NiO
first begins during the production of the diaphragm. As a result, there is a considerable local increase in temperature and the external production temperature can remain at 1000.degree. C., which is advantageous. Furthermore, as a result of the production process, including oxidation-sintering, there is no need to maintain an inert atmosphere. This also signifies a considerable simplification.